VLBA Reveals Closest Pair of Supermassive Black Holes

Astronomers using the National Science Foundation's
Very Long Baseline Array (VLBA) radio telescope have found the closest pair
of supermassive black holes ever discovered in the Universe -- a
duo of monsters that together are more than 150 million times
more massive than the Sun and closer together than the Earth
and the bright star Vega.

The VLBA
CREDIT: NRAO/AUI/NSF

"These two giant black holes are only about 24 light-years
apart, and that's more than 100 times closer than any pair
found before," said Cristina Rodriguez, of the University of
New Mexico (UNM) and Simon Bolivar University in Venezuela.
Black holes are concentrations of mass with gravity so strong
that not even light can escape them.

The black hole pair is in the center of a galaxy called 0402+379,
some 750 million light-years from Earth. Astronomers presume that
each of the supermassive black holes was once at the core of a
separate galaxy, then the two galaxies collided, leaving the black
holes orbiting each other. The black holes orbit each other about
once every 150,000 years, the scientists say.

"If two black holes like these were to collide, that event would
create the type of strong gravitational waves that physicists hope to
detect with instruments now under construction," said Gregory
Taylor, of UNM. The physicists will need to wait, though: the
astronomers calculate that the black holes in 0402+379 won't
collide for about a billion billion years.

"There are some things that might speed that up a little bit,"
Taylor remarked.

An earlier VLBA study of 0402+379, an elliptical galaxy, showed
the pair of radio-wave-emitting objects near its core. Further
studies using the VLBA and the Hobby-Eberly Telescope in Texas,
revealed that the pair of objects is indeed a pair of supermassive
black holes.

"We needed the ultra-sharp radio 'vision' of the VLBA, particularly
at the high radio frequencies of 22 and 43 GigaHertz, to get the
detail needed to show that those objects are a pair of black
holes," Taylor said. The
VLBA is a continent-wide system of ten
radio-telescope antennas. It provides the greatest ability to see
fine detail, called resolving power, of any telescope in astronomy.

"Astronomers have thought for a long time that close pairs of
black holes should result from galaxy collisions," Rodriguez said.
Still, finding them has proven difficult. Until now, the closest
confirmed pairs of supermassive black holes were at least 4,500
light-years apart. Pairs of smaller black holes, each only a few
times the mass of the Sun, have been found in our own Milky Way
Galaxy, but 0402+379 harbors the pair of supermassive black holes
that are the closest to each other yet found.

Galactic collisions are common throughout the Universe, and astronomers
think that the binary pairs of supermassive black holes that result
can have important effects on the subsequent evolution of the galaxies.
In a number of predicted scenarios, such giant pairs of black holes
will themselves collide, sending gravitational waves out through
the Universe. Such gravitational waves could be detected with a proposed
joint space mission between NASA and the European Space Agency, the Laser
Interferometer Space Antenna.

"Such black-hole collisions undoubtedly are important processes, and
we need to understand them. Finding ever-closer pairs of supermassive
black holes is the first step in that process. Even finding one such
system has dramatically changed our expectations, and informed us about
what to look for," Taylor said. Taylor and his collaborators are
currently using the VLBA to carry out the largest survey of compact
radio-emitting objects ever undertaken, in the hope of finding more
such pairs.

Rodriguez and Taylor worked with Robert Zavala of the U.S. Naval
Observatory, Allison Peck of the SubMillimeter Array of the Harvard-
Smithsonian Center for Astrophysics, Lindsey Pollack of the
University of California at Santa Cruz, and Roger Romani of Stanford
University. Their results have been accepted for publication in the
Astrophysical Journal.